This awesome Mark Twain nominee should be the winner. It is a mystery. It is adventure. It is psychological. and it has an alien. In all, it kept my attention to just go on and find out what happens next.
It is set on a moon base in the future. Everything is going boringly well until the death of the base doctor. Dash is convinced that it was murder. No one else is, but almost everyone looks like a suspect. It does get somewhat unbelievable near the end when adults tell him things that they would never actually tell a kid, but it didn't ruin the story.
There is a sequel!